TRANSPORT OF SUPERTHERMAL ELECTRONS IN CORONAL LOOPS AND U(N)-TYPE SOLAR RADIO-BURSTS

Electron beams travelling with about 1/3 of the velocity of light alon g closed coronal loops can manifest themselves in decimeter and meter wave solar type U or U(N) radio bursts. Using a 1-D test particle mode l, we study trajectories of superthermal electrons in coronal loops wi th the aim of understanding recently published detailed radio spectral and imaging data about type U(N) bursts. The computations are carried out in a static semi-circular loop of 1 solar radius length. For mode ling transport processes Coulomb collisions, mirroring of electrons in the loop magnetic field, and scattering in zones of enhanced whistler wave turbulence are taken into account. The formation of a finite zon e of enhanced whistler turbulence in the loop top is consistently expl ained by the properties of loss-cone instability of a weak preexisting energetic particle component. In a model run initially electrons are injected upwards along the loop axis in one leg. Scanning the trajecto ries of electrons through the loop and representing them in space vs t ime and plasma frequency vs time plots, respectively, we get synthetic radio source distributions and radio spectra. The results can be anal yzed in dependence on loop and particle parameters including the stren gth of whistler turbulence. Thus, we are able to model the essential a spects of observed U(N) bursts. We find that in a zone of sufficiently strong whistler turbulence near the loop top the initial electron bea m is splitted up into two beams propagating from the top back and forw ard into both loop legs. Thus two widely separated radio sources brigh ten during the descending branch of U burst spectra. Moreover, we find that U(N) type radio bursts can be excited not only due to mirroring but also by scattering of electrons in whistler turbulence near a leg of the loop. For demonstrating the strength of the present model a sim ulation of an observed U(N) burst (February 23, 1993) is given.